Method for controlling an electric power assist steering system with low hysteresis and torque ripple

ABSTRACT

An electric power assist steering system is controlled by sensing torque in a steering shaft at a point along said steering shaft between a hand wheel and a mechanical connection to an electric motor, wherein the sensing includes sensing a magnetic field direction and intensity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part and claims benefit of U.S.patent application Ser. No. 09/825,794 filed Apr. 4, 2001, now U.S. Pat.No. 6,655,493.

BACKGROUND

In a typical electric power steering (EPS) system, a hand wheel isconnected to a shaft, which comprises an upper shaft and a lower shaftconnected by a torsion bar. The upper shaft connects to the hand wheeland the lower shaft connects to an intermediate shaft that ultimatelyconnects to the rack and pinion gear of a vehicle. When the hand wheelis turned, the upper shaft rotates and a torque sensor measures theangular displacement of the torsion bar. The torque sensor is typicallylocated at the interface between the upper and the lower shaft, which isalso the location of the torsion bar. The type of torque sensortypically used has been a contacting type, which requires use of atorsion bar to measure the amount of twist on the torsion bar. Thetorque sensor sends a signal to the controller, which then sends asignal to the motor to begin operating. The motor powers a gearmechanism, which provides assistance in turning the lower shaft andultimately the road wheels.

A drawback of such torque sensors that rely on the relative rotationaldisplacement of an upper and lower shaft is that they generatehysteresis, which is a lagging effect, and torque ripple, both effectsbeing detrimental to the feel of the power assist steering system.Hysteresis is generated, e.g., from the sensor, the torsion bar itself,bearings on the upper and lower shafts, and any misalignment of theshafts. The amount of hysteresis of the sensor, torsion bar, andbearings can be 0.5 Nm or larger. Hysteresis in these elements generatea torque ripple effect which can be felt at the handwheel as an unevenresistance or periodic pulling effect.

SUMMARY

Disclosed is a method for controlling an electric power assist steeringsystem with low hysteresis and torque ripple by sensing torque in asteering shaft at a point along said steering shaft between a hand wheeland a mechanical connection to an electric motor, wherein the sensingincludes sensing a magnetic field direction and intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several Figures:

FIG. 1 is a schematic perspective view of a steering system of avehicle;

FIG. 2 is a top view of an EPS system with a motor;

FIG. 3 is a cross-section view of an EPS system with a single shaft andsingle housing unit; and

FIG. 4 is a schematic perspective view of a prior art non-complianttorque sensor.

DETAILED DESCRIPTION

Referring to FIG. 1, the steering system 20 comprises an EPS system,which is connected at a hand wheel 24 through a shaft 26 and a housing28. The EPS system provides a driver with assistance in turning avehicle's road wheels 22. The driver turns the hand wheel 24, which ismechanically connected to a shaft 26. The rotational force of the handwheel 24 is transmitted to the shaft 26, which is detected by anon-compliant torque sensor 30. The non-compliant torque sensor 30 islocated at the shaft 26 from about a midpoint 29 at the shaft 26 to anupper end 27 of the shaft 26. The non-compliant torque sensor 30measures the torque applied to the shaft 26 and sends a signal to acontroller 38, which may be a column electronics module. The controller38 then sends a signal to the motor 32 to begin operation. The motor 32,which is in mechanical communication with a worm 34 and a worm gear 36,rotates the worm 34 and the worm gear 36, which provide turningassistance to the shaft 26. As the shaft 26 turns, an intermediate shaft33, connected through a universal joint 31 rotates a pinion gear (notshown) located under a gear housing 35. Rotation of the pinion gear (notshown) moves a rack 41, which moves a tie rod 37. When the tie rod 37moves, it turns a steering knuckle 39, which turns a road wheel 22.

Referring to FIGS. 2 and 3, the EPS system and shaft 26 are mounted to avehicle by a housing 28, which may be a single cast unit. The EPSsystem, shaft 26, and housing 28 collectively may be referred to as thesteering column 60. Referring to FIG. 3, an upper bearing 44 and abearing 46 support the shaft 26. The upper bearing 44 is secured to theshaft 26 by a retaining ring 42. A bearing lash eliminator 48 is pressedbetween the upper bearing 44 and the retaining ring 42.

A position sensor 70, which detects the angular position or displacementof hand wheel 24 (not shown in FIG. 3), is connected to a bracket switchmounting 68, which is in operable communication with the controller 38.The bracket switch mounting 68 is mounted to the face of the housing 28.Both the position sensor 70 and the bracket switch mounting 68 arelocated adjacent to the hand wheel.

As stated above, the non-compliant torque sensor 30 is located anywherefrom about a midpoint 29 at the shaft 26 to an upper end 27 of the shaft26. A spacer 50 may be used to locate the non-compliant torque sensor 30on the shaft 26 in proximity to the end of the controller 38. Thenon-compliant torque sensor 30 comprises a magnetometer housing 52,which is secured to a bearing housing 54 by a fastener 56. The bearinghousing 54 contains a bearing 58 and a bushing 64, which supports themagnetometer housing 52 and secures it to the shaft 26. A snap ring 62secures the bearing housing 54 to the shaft 26. Preferably, there is aconnection pathway 66 in the housing 28 to directly connect thenon-compliant torque sensor 30 to the controller 38, which is located onthe face of the housing 28 adjacent to the hand wheel (not shown).

Referring to FIG. 4, the non-compliant torque sensor 30 comprises atransducer 202 and a magnetic field vector sensor 204. The transducer202 comprises one or more axially distinct, magnetically contiguous,oppositely polarized circumferential bands or regions 206, 208 solelydefining the active or transducer region of the shaft. Region 210 of theshaft to the left of A and region 212 to the right of B aredistinguishable from the active region only by the absence of anysignificant remanent magnetization. The shaft is typically formed of aferromagnetic, magnetostrictive material having a particularly desirablecrystalline structure. When the shaft of the non-compliant torque sensor30 is the shaft 26 of the FIGS. 1-3. torque 214 is applied at oneportion of the shaft 26 and is transmitted thereby to another portion ofthe shaft 26 where the motion of the shaft 26 due to torque 214ultimately turns the road wheels (not shown) of the vehicle. Torque 214is being shown as being in a clockwise direction looking at the visibleend of the shaft 26, but obviously can be applied to rotate in eitherdirection depending on the direction the driver turns the hand wheel(not shown).

A magnetic field vector sensor 204 is a magnetic field vector sensingdevice located and oriented relative to the transducer 202 so as tosense the magnitude and polarity of the field arising in the space aboutthe transducer 202 as a result of the reorientation of the polarizedmagnetization from the quiescent circumferential direction to a more orless steep helical direction. The magnetic field vector sensor 204provides a signal output reflecting the magnitude of torque 214 andelectrically connected to the controller (not shown). The non-complianttorque sensor 30 is more fully described in U.S. Pat. No. 6,145,387,which is incorporated in its entirety herein by reference.

Referring to FIGS. 2 and 3, when the controller 38 receives a signalfrom the non-compliant torque sensor 30 indicating steering effort by adriver against the hand wheel, the controller 38 then sends a signal tothe motor 32 to turn on. When the motor 32 turns on it turns the shaft26 through a worm 34 and worm gear 36 assembly. The worm 34 is rigidlyconnected to a motor 32 and engages worm gear 36. Worm gear 36 ismounted to the shaft 26 on splines (not shown). A spring 74 is mountedbetween the splines (not shown). A nut 72 supports the worm gear 36 inplace along the shaft 26. A bearing 46 supports the worm gear 36 at theshaft 26.

Referring to FIG. 2, a magnetorheological fluid stopper 40 is mounted onthe motor 32. The magnetorheological fluid stopper 40 is fully describedin U.S. application Ser. No. 09/825,793, filed Apr. 4, 2001, entitled,“Magnetorheological Fluid Stopper At Electric Motor” under Attorneydocket number DE3-/DP-303759, which is incorporated in its entiretyherein by reference.

Hysteresis and torque ripple are virtually eliminated by sensing torquein shaft 26 without the use of a torsion bar and improving torque sensoraccuracy and steering accuracy. The elimination of the torsion bar makesunnecessary additional supporting needle bearings, previously requiredto maintain the alignment of shaft portions connected by the torsionbar, further reducing hysteresis.

It will be understood that a person skilled in the art may makemodifications to the preferred embodiment shown herein within the scopeand intent of the claims. While the present invention has been describedas carried out in a specific embodiment thereof, it is not intended tobe limited thereby but is intended to cover the invention broadly withinthe scope and spirit of the claims.

What is claimed is:
 1. A method for controlling an electric power assiststeering system comprising: sensing torque in a steering shaft at apoint along said steering shaft between a hand wheel and a mechanicalconnection to an electric motor, said sensing comprising sensing amagnetic field direction and intensity; detecting said magnetic fieldusing a magnetic field vector sensor disposed coaxially over saidsteering shaft; maintaining said magnetic field vector sensor coaxiallyover said steering shaft by supporting said magnetic field vector sensoron said steering shaft using a bearing; and, prohibiting said magneticfield vector sensor from rotating with said steering shaft bymechanically engaging said magnetic field vector sensor with a steeringshaft housing.
 2. The method of claim 1 wherein said sensing does notinvolve detecting a relative angular displacement between two shaftportions connected by a torsion bar.
 3. The method of claim 1 furthercomprising supporting said steering shaft by a first bearing locatednear a steering wheel at a first end of said shaft and by a secondbearing located near an opposite end of said shaft.
 4. The method ofclaim 3 wherein said supporting comprises supporting said steering shaftonly using said first bearing and said second bearing.
 5. The method ofclaim 1 wherein said sensing includes sensing said magnetic field at aposition along said steering shaft closer to said hand wheel than saidelectric motor.
 6. The method of claim 1 wherein said bearing is a ballbearing.
 7. A method of controlling an electric power assist steeringsystem with reduced hysteresis and torque ripple, the method comprising:sensing torque in a steering shaft at a point along said steering shaftbetween a hand wheel and a mechanical connection to an electric motor,said sensing comprising sensing a magnetic field direction and intensitywithout use of a torsion bar, thereby reducing said hysteresis and saidtorque ripple in said system; detecting said magnetic field using amagnetic field vector sensor disposed coaxially over said steeringshaft; maintaining said magnetic field vector sensor coaxially over saidsteering shaft by supporting said magnetic field vector sensor on saidsteering shaft using a bearing; and, prohibiting said magnetic fieldvector sensor from rotating with said steering shaft by mechanicallyengaging said magnetic field vector sensor with a steering shafthousing.
 8. The method of claim 7 wherein said sensing does not involvedetecting a relative angular displacement between two shaft portionsconnected by a torsion bar.
 9. The method of claim 7 further comprisingsupporting said steering shaft only by a first bearing located near asteering wheel at a first end of said shaft and by a second bearinglocated near an opposite end of said shaft.
 10. The method of claim 7wherein said sensing includes sensing said magnetic field at a positionalong said steering shaft closer to said hand wheel than said electricmotor.
 11. A method for controlling an electric power assist steeringsystem comprising: sensing torque in a steering shaft at a point alongsaid steering shaft between a hand wheel and a mechanical connection toa motor, said sensing comprising sensing a magnetic field direction andintensity with a non-compliant torque sensor; providing a controller inoperable communication with the motor; mechanically engaging said sensorwith a steering shaft housing; connecting said sensor directly to saidcontroller through a connection pathway in said steering shaft housing;receiving a signal from the non-compliant torque sensor in thecontroller; and, sending a signal from the controller to the motor inresponse to the signal from the non-compliant torque sensor received inthe controller.
 12. The method of claim 11 wherein said sensing does notinvolve detecting a relative angular displacement between two shaftportions connected by a torsion bar.
 13. The method of claim 11 furthercomprising mounting said controller on a face of said housing adjacentto said hand wheel.
 14. The method of claim 11 further comprisingmounting a magnetorheological fluid stopper on the motor.
 15. The methodof claim 11 wherein said torque sensor is a magnetic field vectorsensor, the method further comprising disposing the magnetic fieldvector sensor coaxially over said steering shaft.
 16. The method ofclaim 15 further comprising prohibiting said magnetic field vectorsensor from rotating with said steering shaft by mechanically engagingsaid magnetic field vector sensor with the steering shaft housing.